Method of manufacturing Al-Mg-Si alloy sheet capable of forming a flat hemming

ABSTRACT

A method of manufacturing a rolling sheet of AA6000 aluminum alloy comprising Si and Mg as major components is disclosed. More specifically, the present invention provides a method of manufacturing a rolling sheet of an aluminum alloy sheet capable of forming a flat hemming wherein its fractional volume for cube orientation is 35 or higher.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Korean Application No. 2004-0019335, filed on Mar. 22, 2004.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing an AA6000 Al—Mg—Si alloy sheet capable of forming a flat hemming. More particularly, the AA6000 Al—Mg—Si alloy sheet, being capable of forming a flat hemming, can be applied to the manufacturing of a wider scope of automobile parts.

BACKGROUND OF THE INVENTION

Generally, outer body panels of a vehicle require excellent physical properties in formability, shape fixability (a property with high dimensional precision due to low or no springback after press forming), dent-resistance, corrosion-resistance and surface quality. However, the conventional AA5000 Al—Mg alloy sheets have not been favored because they have poor mechanical strength even after press forming and also have poor surface quality. Therefore, the use of AA6000 Al—Mg—Si alloy has been increasingly used lately. The AA6000 Al—Mg—Si alloy provides excellent bake hardenability after painting and high mechanical strength as a result, thus making it possible to manufacture a more thin-gauged and more light-weight sheets.

The vehicle body parts, such as the hood and the like, in general, are manufactured by the mechanical assembly between inner parts and outer parts. For example, the proper length of a flange is prepared and formed at the end of an outer panel of a vehicle. An inner panel is fixed onto the inside of the outer panel, and the flange of the outer panel is bent and folded to produce a mechanical binding. The whole process described above is called “hemming.”

In the hemming process, it is common to perform a flat hemming (180° process), which has very strict processing conditions and a relatively low ratio (r/t) between the bending center radius (r) and the thickness of a sheet (t). However, the bending property of AA6000 Al—Mg—Si alloys is inferior to that of AA5000 alloys. Thus, performing a flat hemming at a part where press property is relatively high (i.e., a part with much transformation) results in a higher defect rate.

Curl hemming, which has been introduced to replace flat hemming to resolve the above problem, is not advantageous in that gaps form in the vehicle body parts and also deteriorates the designs and outer appearances of products manufactured thereof.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a method for manufacturing an AA6000 Al—Mg—Si alloy sheet capable of forming a flat hemming and preventing cracks on such hemmed parts in manufacturing vehicle parts via hot-rolling. The present invention prevents the development of rotated cube-orientation (“RW orientation,” hereinafter) which induces the development of cube orientation that affects the flat hemming on an outer panel. After hot-rolling, the AA6000 Al—Mg—Si alloy sheet undergoes cold-rolling and T4 heat-treatment, thereby securing a particular quality for the outer appearance. Further, the above objective can be achieved when the hot-rolling of the ingot is performed at about 300° C. to 350° C. to limit the development of RW orientation so that its fractional volume is kept at 10 or less and is followed further with cold-rolling and T4 heat-treatment steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a diagram of the texture of an AA6000 Al—Mg—Si alloy sheet after hot-rolling manufactured according to the present invention, a diagram for the texture of the final product, and a picture of the flat hemming process; and

FIG. 2 shows a diagram of the texture of an AA6000 Al—Mg—Si alloy sheet after hot-rolling manufactured according to the conventional method, a diagram for the texture of the final product, and a picture of the flat hemming process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention are described in detail with reference to the accompanying drawings.

The inventors of the invention studied a method to improve the formability of an AA6000 Al—Mg—Si alloy, in particular its bending property, and, after extensive efforts, found that the bending property of the AA6000 Al—Mg—Si alloy is improved according to the localization of cube orientation ({ 100}<100>) of a texture for a given substance. They also found that it is essential to prevent the development of rotated cube-orientation (“RW orientation,” hereinafter), which prevents the development of cube orientation, by optimizing the temperature that influences the friction produced during the hot-rolling process for the optimization of the aforementioned properties.

As stated above, the present invention includes a method for manufacturing an AA6000 Al—Mg—Si alloy sheet with excellent formability capable of forming a flat hemming.

In general, AA6000 Al—Mg—Si alloy sheets are manufactured by undergoing a series of steps in hot-rolling, cold-rolling, and T4 heat-treatment, and, as a result, the final products have crystalline textures comprising copper orientation, Goss orientation, P orientation and cube orientation. Further, the relative fractional volume of each of the above orientations influences the formability of the sheets to be manufactured thereof.

The orientation of a crystal in a sheet manufactured by the rolling process is defined in terms of the surface of a rolling sheet and the orientation of rolling. That is, a texture can be indicated by means of the surface of a crystal located in parallel with the rolling surface and the orientation of a crystal located in parallel with the rolling direction. A specific surface is indicated as {hkl}, and a specific orientation is indicated as <uvw>. Therefore, the following are indicated as follows: copper orientation {112}<111>, Goss orientation {111}<100>, brass orientation {110}<112>, S orientation {123}<634>, cube orientation {001}<100>, RW orientation {001}<110>.

After cold-rolling, the texture of the sheet comprises copper orientation, brass orientation, and S orientation, whereas the texture changes after T4 heat-treatment to comprise cube orientation, S orientation and brass orientation. Further, a poor level of Goss orientation is developed. Of these orientations, cube orientation is the most significant factor that influences the formability of the sheet.

To prevent the development of cube orientation, the introduction of an additional process for production, such as rolling shear, is required.

To improve the flat hemming, it is necessary to develop the cube orientation. However, in the event that the development is primarily directed toward S and P orientations, it results in the prevention of the cube orientation development, thereby deteriorating the flat hemming. Therefore, it is necessary to suppress the development of S and P orientations in this case.

Yet, S orientation is an orientation that develops as a result of cold-rolling, and it is impossible to prevent the development of this orientation. Further, P orientation is an orientation that develops as a result of adding Mg and Si, and its development cannot be prevented.

In the event that the development of cube orientation is expedited during the T4 heat-treatment, the development and growth of the cube orientation can either decrease or suppress the crystal grain fractions for the orientation.

To develop the cube orientation, it is necessary to develop a copper-type texture during the cold-rolling process. This is because the cube-oriented grain tends to grow into the neighboring grains, which are copper-type orientations (A. A. Ridha and W. B. Hutchinson, Acta metall., Vol.30, pp.1929-1939, 1982). However, when the copper-type texture is developed, it cannot prevent the presence of S orientation, and thus the development of cube orientation can be relatively mitigated, which then results in the deterioration of the flat hemming.

In the present invention, a copper-type texture is developed after cold-rolling, and thus a cube orientation can be developed after T4 heat-treatment. Further, this invention suggests that RW orientation can be suppressed so that the development of S orientation can be prevented and the development of cube orientation is controlled to have a fractional volume of 35 or greater.

RW orientation is an orientation that develops as a result of shear deformation due to friction and the like from hot-rolling. RW orientation can expedite the development of S orientation of copper-type orientations if it is passed through the cold-rolling process. It can also suppress the development of cube orientation during the recrystallization heat-treatment (i.e., T4 heat-treatment), and thus preliminary suppression of the RW orientation development can lead to the desirable development of cube orientation.

In one embodiment, the present invention provides a method for manufacturing an 6000 Al—Mg—Si alloy sheet, wherein the hot-rolling process is performed at about 300° C. to 350° C. instead of the conventional hot-rolling performed at 300-400° C., thereby reducing the friction between a given sheet and the roll. As a result, the development of RW orientation is prevented, which then leads to the cube orientation development so that a fractional volume of 35 or greater in the 6000 Al—Mg—Si alloy sheet can be prepared eventually after being passed through the cold-rolling and T4 heat-treatment processes, whereby providing the alloy sheet with excellent formability and capability of forming a flat hemming.

Preferred embodiments of the invention are given below with reference to the accompanying drawings.

An Al—Mg—Si alloy was prepared using the conventional DC casting method to have the thickness of 120 mm, homogenized at 480° C. for 48 hours and then processed under the hot-rolling step until it has a thickness of 5 mm. The initial temperature for hot-rolling was from about 300° C. to 350° C. and its coiling temperature was fixed at 300° C. The above-prepared material was treated with cold-rolling to have a thickness of 1 mm and further treated with T4 heat-treatment, which comprises solution treatment and a quenching process.

Flat hemming was performed on the above-prepared sheet, where there was a 10% tensile deformation, and the presence of cracks on the sheet's surface was observed. The presence of cracks after flat hemming was performed on the specimens prepared according to the present invention and the conventional method are shown in FIGS. 1 and 2, respectively.

The level of RW orientation and cube orientation development after hot-rolling and T4 heat-treatment were compared by using the orientation distribution function (ODF) and are respectively shown in FIGS. 1 and 2. In the samples manufactured according to the present invention, the RW orientation was suppressed as shown in FIG. 1, whereas the development of cube orientation was more than doubled as shown in FIG. 2. The ODF enables quantitative analysis of the level of development of a given texture. The contour line indicates the level of development of a given texture, wherein the higher the contour line the higher the level of development of a texture.

As shown in FIG. 1, flat hemming is made possible as the cube orientation is developed. However, in the samples prepared according to the conventional method, cracks were observed during the flat hemming process. TABLE 1 Vol. Hot-Rolling Fraction of Sample Initial Coiling RW Cube Presence No. Temp. (° C.) Temp (° C.) Orientation Orientation of Cracks Remarks 1 350 300 8.5 40.0 No *P 2 340 300 7.8 45.3 No *P 3 330 300 7.8 49.2 No *P 4 350 320 10.0 35.6 No *P 5 320 300 6.3 50.3 No *P 6 450 310 28.8 13.0 Yes *C 7 360 300 17.0 25.0 Yes Com. Ex. 8 360 320 19.1 23.0 Yes Com. Ex. 9 360 340 21.0 22.7 Yes Com. Ex. 10 320 295 Strip — — Com. Ex. Breakage 11 290 250 7.1 21.0 Yes Com. Ex. (*P: the method of the present invention; *P: the conventional method; Com. Ex.: Comparative Example)

The above Table 1 shows the effects of the present invention by comparing the method of the present invention and the conventional method. The hot-rolling conditions of the present invention is shown in Table 1, and the processes to be followed thereafter are cold-rolling and T4 heat-treatment, which are the same as in the conventional method.

As for the hot-rolling temperature in the conventional method, the initial temperature is 450° C. and coiling is performed at 310° C. To eliminate the work hardening effect during cold-rolling, both samples prepared by the present invention and the conventional method underwent T4 heat-treatment.

Table 1 shows that cracks are not generated after flat hemming when hot-rolling is performed at a temperature range of about 300° C. to 350° C. as in the present invention because the fractional volume of RW orientation is 35 or higher. This is because there is less friction between the sheet and the roll in the temperature range of about 300° C. to 350° C., thus reducing shear deformation whereby the development of RW orientation is prevented to a range of 10 or less.

In contrast, in the samples prepared according to the conventional method, the cube orientation development was not remarkable but had a fractional volume of 28 that resulted in the generation of cracks during the flat hemming process.

In Sample 10, strip breakage occurred, making the subsequent effects not observable.

In Samples 7-9, in which temperature conditions were out of the range of the present invention, the RW orientation development was expedited due to the adhesion friction between the starting material and the roll during the hot-rolling process, thereby generating cracks in the final product during the flat hemming process.

In Sample 11, the adhesion friction had no effect, however, the relatively low working temperature resulted in an increase in the rolling pass from the usual 8 passes to 13 passes, thus increasing unit cost for production. Nevertheless, the cube orientation development was not induced.

While the foregoing description represents various embodiments of the present invention, it will be appreciated that the foregoing description should not be deemed limiting since additions, variations, modifications and substitutions may be made without departing from the spirit and scope of the present invention. It will be clear to one of skill in the art that the present invention may be embodied in other forms, structures, arrangements, and proportions, and may use other elements, materials and components. The present disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and not limited to the foregoing description. 

1. A method of manufacturing Al—Mg—Si alloy sheets capable of forming a flat hemming comprising hot-rolling an Al—Mg—Si alloy sheet at a temperature range of about 300° C. to about 350° C.
 2. The method of claim 1, wherein said method further comprises a cold-rolling and T4 heat-treating step following said hot-rolling process.
 3. The method of claim 2, wherein said cold-rolling is performed until said alloy sheet develops a thickness of 1 mm.
 4. The method of claim 2, wherein said heat-treating comprises a T4 heat-treatment.
 5. A method of manufacturing alloy sheets capable of forming a flat hemming, comprising: preventing a development of a rotated cube orientation during manufacturing of an alloy sheet; cold-rolling said alloy sheet; and heat-treating said alloy sheet.
 6. The method of claim 5, wherein said alloy sheet comprises an Al—Mg—Si alloy sheet.
 7. The method of claim 5, where said preventing comprises reducing friction between said alloy sheet and a roller used during a hot-rolling step, wherein said hot-rolling step is performed at a temperature range of about 300 to 350° C.
 8. The method of claim 7, wherein said hot-rolling step is performed until said alloy sheet develops a thickness of 5 mm.
 9. The method of claim 7, wherein said hot-rolling step further comprises a coiling temperature of 300° C.
 10. The method of claim 5, wherein said cold-rolling is performed until said alloy sheet develops a thickness of 1 mm.
 11. The method of claim 5 wherein said heat-treating comprises a T4 heat-treatment.
 12. The method of claim 1 1, wherein said T4 heat-treatment further comprises a solution treatment and a quenching process.
 13. A method of manufacturing alloy sheets capable of forming a flat hemming, comprising: hot-rolling said sheet at a temperature range of about 300° C. to about 350° C.; cold-rolling said sheet; and heat treating said sheet.
 14. The method of claim 13, wherein said alloy sheet comprises an Al—Mg—Si sheet. 